ABSTRACT This is an application by Mortimer Poncz, MD, for an NHLBI-supported R35 Outstanding Investigator Award (OIA). Dr. Poncz has made pioneering contributions to the field of platelet (Plt) biology. He isolated and charac- terized the first cDNAs and genes for the Plt proteins (integrin chains, aIIb and b3, and the chemokines, platelet factor 4 (PF4) and b-thromboglobulin). He characterized the first molecular defects in an inherited Plt disorder, Glanzmann thrombasthenia (GT). For over 30 years, Dr. Poncz has made insightful contributions to move the Plt field forward. He helped advance our understanding of megakaryopoiesis, defining many of the transcriptional factors (TFs) fundamental to that process and their role in inherited Plt disorders. He pioneered the use of induced pluripotent stem cells (iPSCs) to study megakaryocyte (Meg) biology and demonstrated the first strategy for genetically correcting the defect in GT and in Paris-Trousseau syndrome. Dr. Poncz proposes to pursue new insights into the molecular basis of the thrombocytopenia observed in RUNX1 haploinsufficiency, and how this intervention might decrease the risk of leukemic transformation in affected patients. This interest in megakaryopoiesis also lead to Dr. Poncz?s contribution to understanding where Plts are released, showing that infused Megs release Plts in the lungs. The resulting Plts are much more physiologic than any ex vivo-generated Plts released from in vitro-grown Megs to date. These studies supported a potential pulmonary site for a portion of thrombopoiesis with subsequent studies by others providing in situ-support for this model. Dr. Poncz now shows that the lung microcapillary bed is unique in being able to release Plts. Proposed studies will further our understanding of what makes the pulmonary bed unique for thrombopoiesis, and such insights may have clinical application in Plt transfusions. Dr. Poncz also proposed that Plts could store ectopic proteins in a granules, releasing them in a targeted, potent fashion at sites of vascular injury. He proposes advancing the use of Plt-targeted therapeutics by a novel new mechanism for loading such proteins into Plts to treat hemophilia A patients with inhibitors with Factor VIII variants and for use as a thromboprophylaxis agent with urokinase variants. Dr. Poncz has also advanced our understanding of the molecular basis of prothrombotic heparin-induced thrombocytopenia (HIT). His HIT-like antibody KKO and murine model for HIT are widely used in the field and both were used in his recent advances in understanding the role of neutrophil extracellular traps (NETs) in HIT. He now proposes that polyanions like NETs underlie the prothrombotic nature of HIT, and will test this in this R35. Moreover, he believes that these new insights have implications in the care of other thromboinflammatory diseases where NETs have an important role, and proposes use of PF4 and Fc-modified KKO in the treatment of sepsis and sickle cell disease. Support from this R35 OIA mechanism will also provide Dr. Poncz with the time to pursue his other passion of advancing the career of mentees in areas related to benign hematology.